Metal nanoparticles have unique physical and chemical properties, and industrial application of such particles is therefore attracting considerable attention. A variety of methods have been proposed as production methods for metal nanoparticles. These methods can be broadly classified into wet methods and dry methods, wherein a representative wet method is a method in which a salt or complex of a metal is reduced in solution by a co-existent reducing agent, and a representative dry method is a metal ingot gas evaporation method (Non-Patent Document 1). Among the various metal nanoparticles, although it is known that alloy nanoparticles, and particularly nanoparticles of alloys of a noble metal such as platinum, palladium, gold, silver, rhodium, ruthenium or iridium, and a base metal such as iron, cobalt, nickel, copper or chromium, have considerable practical significance in terms of their catalytic action and their electromagnetic properties or optical properties, the production of these alloy nanoparticles requires a multi-stage operation, and no simple production method is known. For example, solid solution alloys such as platinum-cobalt, platinum-nickel, platinum-iron and platinum-cobalt-chromium alloys have a high electrochemical oxygen reduction mass activity, and are useful as the active species of the cathode catalyst for a fuel cell, but the preparation includes, for example, causing a salt of a base metal such as cobalt to act upon platinum nanoparticles supported on a carbon carrier, subsequently performing either a neutralization to support the base metal as a hydroxide or a treatment with a reducing agent such as hydrazine to support the base metal as a metal, and then conducting further treatment at a high temperature of 800 to 900° C. that involves either performing a hydrogen reduction-alloying treatment or performing an alloying treatment under a stream of argon to form the alloy catalyst. As a result of this high-temperature heat treatment, the alloy crystallite size is typically 5 nm or greater, and obtaining a fine alloy catalyst with a crystallite size of not more than 3 nm has proven problematic (for example, Patent Document 1).
On the other hand, ordered alloy nanoparticles of platinum-iron or platinum-cobalt have a high magnetic anisotropy, and are therefore attracting much attention as high-density magnetic recording materials, but production of these alloys involves first preparing disordered alloy nanoparticles using a polyol reduction method within a high-temperature organic solvent of 260 to 300° C., subsequently supporting these nanoparticles on a substrate, and then conducting a second heat treatment at a high temperature of 500° C. or higher to finally obtain the targeted ordered alloy nanoparticles (Patent Document 2, Non-Patent Document 2). Because of this ordering treatment, fine ordered alloy nanoparticles having a particle size of less than 3 nm, such as those obtained using a wet nanoparticle production method, can not be obtained.
On the other hand, some prior techniques relating to methods that utilize laser light in the production of nanoparticles are already known. A method has been reported in which a gas containing carbonyl compounds of two different transition metals such as Fe and Co or Fe and Cr is irradiated with laser light, thereby yielding a γ-phase alloy, which is a high-temperature crystalline layer, in the form of microparticles, a powder or a thin film (Patent Document 3). This method is suitable for metals having compounds that can be gasified, but is difficult to apply to heavy elements such as platinum or palladium for which gasification is problematic.
A production method has been reported in which a coating solution comprising precursor disordered alloy nanoparticles is applied to a support, and the resulting coating film is then irradiated with laser light, thereby forming CuAu-type or Cu3Au-type hard magnetic ordered nanoparticles (Patent Document 4). Furthermore, a production method for magnetic nanoparticles has been reported in which amorphous nanoparticles protected with organic ligands are prepared and purified in advance using a wet method such as a hot soap method, and these nanoparticles that have been stabilized by the organic ligands are then irradiated with laser light to effect a crystallization (Patent Document 5).
In order to obtain the targeted crystalline alloy nanoparticles, these conventional production methods require at least two stages, or as many as 3 to 5 stages if intermediate purification steps such as extractions or drying steps are also included, and as such, are long and complex processes that incur large losses in terms of resources and energy due to the heating and washing steps conducted using large volumes of solvent. Furthermore, because of the latter-half heat treatment, aggregation or particle size growth of the microparticles generated in the preceding steps is unavoidable.
Furthermore, methods for forming microparticles of metals or metal oxides using conventional laser ablation methods are widely known (for example, Patent Document 6, Patent Document 7, and Non-Patent Document 3). These are methods in which a solid metal raw material in a liquid phase is irradiated with laser light, thereby yielding microparticles having a smaller particle size than the raw material, and as such, are not bottom-up methods in which nanoparticles are formed from molecules, but so-called top-down methods in which solid agglomerates are reduced in size. These methods suffer various problems, including there being a limit to the degree of size reduction, and the fact that controlling the particle size distribution is not easy.
Meanwhile, in the case of Pt—Fe alloy, which is recognized as one of the alloys with the highest magnetic anisotropy among currently known magnetic materials, the smallest particle size that has been reported to date is 3 to 4 nm. Furthermore, as the particle size of ferromagnetic particles is reduced, the superparamagnetic critical size, at which the exchange interactions between electron spins lose out to thermal disturbances, leading to a loss of ferromagnetism, has been reported as approximately 3 nm at 300 K in the case of a Pt—Fe alloy (Non-Patent Document 4).
In other words, for future ultra high-density magnetic recording element applications, ferromagnetic nanoparticles that are as fine as possible, for example 3 nm or less, are required, but no production method has been established that is effective in producing such ultra fine ordered alloy nanoparticles, and even if such a method were to exist, it had been thought that the wall of superparamagnetism would be an impediment to achieving ferromagnetism within the normal temperature region.    Patent Document 1: Japanese Laid-open publication (kokai) No. 2000-323145    Patent Document 2: U.S. Pat. No. 6,254,662    Patent Document 3: Japanese Patent Publication No. 3,268,793, Japanese Laid-open publication (kokai) No. Hei 5-65512    Patent Document 4: Japanese Laid-open publication (kokai) No. 2003-260409    Patent Document 5: Japanese Laid-open publication (kokai) No. 2005-48213    Patent Document 6: Japanese Laid-open publication (kokai) No. 2005-272864    Patent Document 7: Japanese Laid-open publication (kokai) No. 2003-306319    Non-Patent Document 1: Masaaki Oda, Ultra Fine Particles, edited by Hayashi, Ueda and Tazaki, 115, 1988, published by Mita Shuppan    Non-Patent Document 2: S. Sun et al., Science, vol. 287, 1989, 17 Mar. 2000    Non-Patent Document 3: S. Koda et al., J. Phys. Chem. B, 103, p 1226 to 1232 (1999)    Non-Patent Document 4: M. Watanabe et al., Mater. Trans. JIM, vol. 37, 489, 1996